Andrew Kesterson
91fadf032b
Eleven of the thirteen defects these two games found are fixed, and the chapters that taught around them said things that are no longer true. Chapter 17: the drawing verbs are no longer invisible, they are covered by a text layer that `WINDOW` can now shrink; the cell size is `RGR(3)` and the grid is `RWINDOW`, not a hand-measured constant; a character written past a short row's end lands. Chapter 18: traps 3 and 4 -- the skipped block that broke its caller's `RETURN`, and `SSHAPE` ignoring a subscript -- become history rather than warnings, and Step 3 no longer claims a sprite is the *only* way to put a picture up, only the one that costs nothing. **Neither `.bas` listing changes, and both READMEs now say why.** The character game still writes `CW# = 16` and the artwork game still keeps six brick stamps in six scalars. What those listings are worth is being what a program written against those constraints looks like, with comments explaining what each one was avoiding -- rewriting them to pretend the problems never existed would throw that away. So Chapter 17 Step 2 shows the `RGR(3)`/`RWINDOW` form and says plainly that the listing beside it does not use it. That is the one place in either chapter where a quoted fragment is not verbatim from the game. Also: the closing pointers now say which entries are struck and which stand, and the budgets table in Chapter 18 notes that several of those budgets were tighter when the game was written. TODO.md section 9 item 3 is updated rather than struck: the `WINDOW` half is fixed, the default text area owning the whole window is not, and whether that default is right is a question rather than a defect. Verified against the fixed interpreter: both games run 45 seconds on the SDL frontend with no error line and the score climbing; both suites are green in both configurations; `docs_examples` passes and `docs_screenshots --check` re-renders all thirteen figures byte-identically; coverage is 95.0% of lines against the 90 gate, with src/variable.c at 100%; and every other quoted fragment still appears verbatim in the listing it came from. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
4.2 KiB
Breakout
Breakout, written entirely in BASIC for the AKGL BASIC interpreter. The ball and the
paddle are sprites defined from DATA in the listing; the bricks, the HUD and the
messages are characters in the text grid.
../../../build-akgl/basic breakout.bas
It needs the SDL build. The stdio build has no graphics device, so RGR refuses on the
fourth line and the game stops there — which is exactly what it should do.
| Key | Does |
|---|---|
| left / right | move the paddle |
| space | start a game from the title screen, then launch the ball |
| P | pause |
| Q or escape | quit |
The title screen plays itself. Attract mode is a real feature and it is also how the game was tested without a hand on the keyboard.
Three lives, six rows of bricks worth 60 down to 10 points a row, three level layouts that repeat with a faster ball each time, and a high score that lasts as long as the process does.
How it works, and how to write your own
Chapter 17 of the guide builds this program a step at a time: what to make a sprite and what to make a character, why every name is declared before the game starts, how a text row has to be written whole, and the rest of the rules this listing never breaks. It is the tutorial; this is the finished thing.
Chapter 18 does the same for
../sprites, which builds the same game out of loaded artwork and comes out
a completely different program.
Known warts, this game's own
- The cell size is a constant.
CW#andCH#are 16, measured with the bundled C64_Pro_Mono at 16 points in the 800×600 window; the grid is 50×37. When this was written BASIC had no way to ask. It has now —RGR(3),RGR(4)andRWINDOW, added because of this listing — and the listing has not been changed to use them, because what it demonstrates is what a program does when the interpreter will not tell it. - Every character the game draws also goes to stdout, because the frontend tees the text sink to the terminal. It made the whole game auditable from a log file during development and it is pure noise the rest of the time. Redirect it.
- Collision is against the cell the ball's leading edge is in, so a brick clipped at the very corner can be missed by up to seven pixels' worth of ball. Testing the centre instead was worse: the ball sank four pixels into a brick before it turned.
- There is a stall watchdog. Ten seconds without touching a brick and the ball gets a new angle — at the next paddle bounce, never in mid-air.
- The demo cannot lose, it re-serves. It also cannot be paused;
Pis ignored until a real game starts. - Sound is asked for once and then believed. On a machine with no audio device the game is silent rather than dead.
- No high score on disk. There is no disk in this interpreter.
The interpreter defects this game turned up are filed in TODO.md §6 and §9, each with a
reduction that fits on a screen. Most are now fixed — the value pool that killed the first
version after twenty-five seconds, the unreachable WINDOW, the character written past a
short row — and the entries are struck rather than deleted, because the reduction is worth
keeping. The listing itself is unchanged, and deliberately: it is what a program
written against those constraints looks like, and its comments explain what it was
working around.
How this was checked
- Parses clean under the stdio
basic(it then stops at the first graphics verb, as it should). - Attract mode run for 145 seconds and again for 110: no runtime error, score climbing throughout, 39 bricks in the second run.
- Level clear verified against a copy whose first layout is a single row:
LEVEL CLEARED, layout 2 loaded, ball speed up, play continues. - Input driven with
xdotoolagainst the real window: paddle moves and clamps at both edges, ball launches,Ppauses and unpauses, three lives drain toGAME OVER, space starts a new game,Qexits printing the final and high scores. - Every direction change logged for a whole run and read back: each one is a wall, a brick or the paddle. Nothing turns without a reason.